Ta is one of the best hard masks for MTJ reactive ion etching (RIE) due to a very low etching rate using Methanol and high etching selectivity to MTJ materials. Usually, a Ta hard mask is patterned by a dielectric hard mask, for example, SiO2, SiN, APF (advanced patterning film), etc., and the dielectric hard mask is patterned by photoresist (PR). This is because there will be a fencing issue if PR is used directly to pattern Ta. Serving as the MTJ etching hard mask and top electrode, we need Ta to be thicker than about 38 nm. However, we always need to balance between high selectivity for both PR/SiO2 and Ta/SiO2, and sidewall striation, which usually happens during high selectivity etching schemes and results in non-uniform MTJ devices.
Several patents show the use of various hard masks in etching MTJ stacks. These include U.S. Pat. No. 8,722,543 (Belen et al), U.S. Pat. No. 8,324,698 (Zhong et al), U.S. Pat. No. 8,133,745 (Zhong et al), and U.S. Pat. No. 7,001,783 (Costrini et al) and US Patent Application 2012/0028373 (Belen et al).